Capacitive sensor systems are well known for their capability of providing control and operation of different appliances based on the presence of a human body. Such systems in general are for instance described in U.S. Pat. No. 4,453,112 and U.S. Pat. No. 5,621,290. In these documents a sensor electrode is arranged on the window frame of a car window. As soon as a portion of a human body, such as a hand, approaches the sensor electrode, the capacitance between the sensor electrode and an earth electrode increases. This increase in capacitance changes the frequency of an output signal of the electrode, which is compared to a reference, and a motor moving the window operates based on this change. The sensor system can respond to semi-conductive elements such as a human body, but plastics and wood will not cause any effect.
Another system providing a capacitive sensor capability is disclosed in document WO02089328 in which the sensor circuit is connected to a load line providing power to an electrical device, such as an electrical motor. This system provides the benefit that the load line instead of influencing the sensor system and causing a decrease in sensor capability near that line, instead will support the system by operating as an antenna part of the sensor system.
It is sometimes interesting to arrange the antenna of the capacitive sensor system on the part which is moving, such as a door, lid or window as described above. Such a system is for instance disclosed in the document WO03069104, in which the capacitive sensor antenna is positioned to avoid that the automatic elevator door pinches a person. When someone puts his hand near the antenna the door will be stopped to prevent accidents to happen.
One problem with this last mentioned system and other similar systems with an antenna positioned on the moving part is that when the door or the like moves towards a closing position the metal frame which is normally a part of the construction holding the door will interfere with the system. The reason for this is that the door frame is a part of the earth plane that in said system forms a first electrode of the capacitance of said sensor. The antenna positioned on the door forms the second electrode. The system will therefore not be able to detect a human body part positioned at the frame. When the door is closing, the system will sense the increase in the capacitance value caused by the approaching door frame. However, to avoid that this influences the system, a default set will normally make sure that this change will not cause the door to be stopped.
Such a default set will result in that a human body part, especially a smaller one, will not be detected by the system. The frame will make the system blind since its major influence on the capacitance is much larger that the smaller influence on the capacitance caused by the body part.